A multi-component fiber is a fiber consisting of at least two components; various types are known. Specifically these include the sheath-core type composite fiber as shown, for example, in U.S. Pat. No. 2,989,798 and British patent No. 514,638, the so-called side-by-side type composite fiber as shown in U.S. Pat. Nos. 2,428,046, 3,038,239 and 3,117,906, the so-called kidney type composite fiber as shown in U.S. Pat. Nos. 2,987,797 and 3,035,235 the "islands-in-a-sea" type composite fiber as shown in British patent No. 1,171,843, the composite fibers having irregularly shaped cores as shown in U.S. Pat. Nos. 3,350,488 and 2,932,079 and 3,672,802 and in French patent No. 1,495,835 and polymer blend type fibers as shown in U.S. Pat. No. 3,099,067, for example. Further, as a special example, a polymer blend may be used as a component of an islands-in-a-sea type composite fiber.
Of these multi-component fibers, typical examples of cross-sections of islands-in-a-sea type composite fibers are shown in FIGS. 1 (a) and 1 (b). FIG. 1 (a) is an example of an islands-in-a-sea type composite fiber having 16 islands, whereas the fiber of FIG. 1 (b) has 36 islands. In both, some islands are surrounded by other islands. When the number of islands in the fiber is increased, this increases the ratio of islands to the sea, and the stability of the fiber increases.
These multi-component fibers have many uses per se. However, by removing at least one component from such a fiber, a unique usage is opened up, as in the case of, for example, British patent No. 1,218,191. For example, a component may be removed by dissolution of a conventional multi-component fiber, but this presents problems.
The following characteristics are often required of a component to be removed by dissolution:
1. Good spinnability; it is intended to be spun together with one or more other components; it must be stably spinnable at the existing spinning temperature.
2. It must not react with other components during spinning (especially, it must not gel by any cross-linking reaction).
3. It must be drawable (it must not fuse during drawing).,
4. It must have flexibility to some extent (which is especially necessary when it is to be crimped).
5. It must be readily soluble.
6. It must be low in cost.
The following are examples of conventional components to be removed by dissolution:
A. Examples of components having good spinnability, drawability and flexibility while sacrificing solubility:
Polyamides, polyesters and polyacrylonitriles are excellent in respect of spinnability, but are difficult to remove by dissolution, as has been observed in U.S. Pat. Nos. 3,350,488 and 3,382,305 and in French patent No. 1,495,835.
In case (A), with respect to the solvent, there are problems such as dissolving speed and difficult handling. For example, when dissolving polyamide in formic acid, the material of the container and the design of the machine for handling formic acid present industrial problems. Almost no materials satisfactorily reist corrosion by formic acid except titanium alloys, especially when heating is used for increasing solubility. Especially when removal of formic acid from the product after dissolution, and recovery of formic acid are taken into account, the use of formic acid on an industrial scale is very troublesome.
In the use of ortho-chlorophenol as a solvent for polyester also, danger of using the solvent is great and its dissolving speed is too slow. In the case of acrylonitrile, there is only a limited selection of polymers which can be simultaneously spun with acrylonitrile. Also great difficulty is encountered in its removal by dissolution.
B. Examples of components having good solubility while sacrificing spinnability;
Some polymers are inferior in drawability and flexibility, such as polystyrene, polystyrene-acrylonitrile copolymers and polystyrene-methyl methacrylate copolymers, as reported in British patent No. 1,263,221 and in U.S. Pat. No. 2,930,074.
In case (B), solvent which is low in cost and easy to handle, such as a hydrocarbon of the aromatic series or a hydrocarbon of the chlorine series may be selected as a solvent.
However, when such a polymer is used, especially when such a polymer occupies at least 40% of the surfaces of the multi-component fiber, the drawability and flexibility of the fiber become very poor. The following explanation will elaborate.
Drawbacks in the use of polymers of the polystyrene series:
Polystyrene per se is a very brittle polymer; the elongation of undrawn polystyrene yarn at room temperature is at most 6 - 10%. Because it is brittle, polystryrene alone is very difficult to draw. By using polystyrene as one component of multi-component fiber, the polystyrene is reinforced by other components and becomes somewhat easier to handle. However, the other components become weaker because of the presence of the polystyrene, which is most difficult to draw. Especially when polystyrene is present over at least 40% of the surface of the multi-component fiber, the fiber becomes most difficult to draw. When the temperature is raised so that the polystyrene can be drawn, the polystyrene becomes tacky and the multi-component fibers stick to one another.
In a multi-component fiber, when polystyrene occupies at least 40% of the fiber surface, it is very difficult to draw the fiber. Polystyrene begins to flow at a temperature of 105.degree. - 115.degree. C. As soon as the polystyrene begins to flow, the multi-component fiber becomes tacky, the fiber fuses on the surface of the heat source (hot plate) or the fibers fuse among themselves. Also, the fiber can be drawn only within a very limited range and for a very short period of time. It may be stated, accordingly, that such fibers cannot be drawn sufficiently, in the industrial sense. Elongation at a lower temperature does not give sufficient results; for example, by drawing with steam heat as is ordinarily carried out industrially in staple drawing, the multi-component fibers can be drawn only to 2.0 - 2.5 times its initial length. When drawn more, the polystyrene on the surface whitens, cracks and breaks, and the resulting fiber cannot withstand actual use.
Specifically, since it is impossible sufficiently to draw such a multi-component fiber under normal industrial conditions, this makes it necessary to make a fine denier undrawn yarn, in which case spinning productively suffers.
Because a complicated spinneret is usually used to obtain a multi-component fiber, it is difficult to effectively increase the number of nozzles on the spinneret. Accordingly, spinning productivity cannot be increased which is a fatal drawback with respect to the cost of the fiber as a product.
Further, the physical properties of the multi-component fiber product also deteriorate. Because the fiber is not to be drawn to the desired extent, its elongation is quite high and its Young's modulus is low. The characteristics of the fiber are close to those of undrawn yarn. Such poor drawability and brittleness of polystyrene are particularly troublesome when a highly shrinkable fiber is desired.
In order to obtain a highly contractible fiber, the fiber must be drawn at as low a temperature as possible so as to impart an internal strain to the fiber. When polystyrene is used as the sea component in an islands-in-a-sea type composite fiber, such fiber cannot be drawn at as low a temperature as 60.degree. - 70.degree. C; at 98% C or higher drawability begins to some extent, but the resulting fiber does not have a sufficient (e.g. more than 25%) shrinkage.
And in the case of a contractible fiber, two-stage contractibility is important. When the fiber has been once contracted at a relatively low temperature, and is thereafter contracted at a higher temperature, the fiber should still show contractibility. The most ideal relationship in two-stage contractibility is that the sum of the first stage shrinkage and the second stage shrinkage equals the shrinkage that would be obtained if the fiber were suddenly exposed to the higher temperature.
When the fiber is drawn at a high temperature, it is difficult to obtain a fiber having excellent two-stage contractibility. When polystyrene is used, the temperature at which the drawn yarn begins to contract is relatively high, due partly to the limiting condition that polystyrene must be drawn at a high temperature. It is difficult to carry out stepwise slow contraction on the low temperature side, it is not possible to provide a wide range of contracting temperature upon carrying out two-stage contraction, and two-stage contraction is accordingly difficult.
As a means for solving the aforementioned problems of drawability and contractibility when polystyrene is used, it is conceivable to add a plasticizer to the polystyrene. However, no anticipated substantial effect is obtained. Further problems arise when a plasticizer is used, including mixing and affinity of the plasticizer with the polymer, bleed-out and evaporation of the plasticizer.
In general, plasticizing of a polymer with a plasticizer requires a large amount of the plasticizer. When a large amount of plasticizer is added to the polymer, it is technically difficult to mix the two uniformly, and even if they are mixed, the plasticizer bleeds out and evaporates through the fiber surface during melt spinning of the polymer. The amount of plasticizer remaining in the polymer fiber is sharply reduced and a substantial plasticizing effect cannot be realized.
Further, when such a large amount of the plasticizer is added to the polymer, the melt viscosity of the polymer becomes drastically lower. In the spinning of multi-component fibers, the maintenance of a balance of melt viscosity values among respective components is very important for the stabilization of spinning. Poor balance results in composite unevenness and abnormal variations of cross-section and bending of the fiber just under the spinneret.
Further, crystallization of other polymers is sometimes caused by the plasticizer. When crystallization proceeds in an undrawn yarn, the fiber becomes difficult to draw. Depending upon the particular plasticizer, such crystallization may even be accelerated.
Also, with reference to a problem of evaporation of the plasticizer at the time of spinning, the plasticizer tends to evaporate through the surface of a yarn having a large surface area and the plasticizer tends to form bubbles, which tend to cause breakage of the yarn.
As mentioned above, improvement of drawability by adding a plasticizer can hardly be expected. Accordingly, it is an object of great desirability to develop a novel polymer. Such a novel polymer should be easy to draw at a relatively low temperature, and should not fuse within at least a certain range of temperature within which other components are drawn. In the case of polystyrene, it fuses simultaneously under normal drawing conditions. In the case of polystyrene-acrylonitrile copolymers and polystyrene-methyl methacrylate copolymers, they also present the same problems associated with polystyrene and they do not show any improvement of drawability of flexibility.